Process of impregnating a microporous article



Patented July 20, 1 954 PROCESS OF IMPREGNATING A MICROPOROUS ARTICLE Gordon W. Knapman, Beverly Hills, and Maynard Jordan Nathason, Pasadena, Calif.

No Drawing. Application November 13, 1951, Serial No. 256,164

1 Claim. 1

This invention relates to the impregnation of materials with a liquid impregnant for the pur pose of preserving or water-proofing the impregnated materials or of bonding the materials, and more particularly relates to a process of impregnation with materials which substantially preserve their liquidity until subjected to heattreatment, of which thermosetting copolymers form an important example. While a major use of the process is the impregnation of metals, both ferrous and non-ferrous, the process may also be used to impregnate glass, hard plastics, hard rubber, neoprene, and similar materials to cause an intimate and strong bonding of such materials, either to one another or to metals.

This application is a continuation in part, as to all common subjectmatter, of our copending application of the same title, Serial No. 148,004, filed March 6, 1950, now abandoned. This present application amplifies some of the details of that parent case, and supports some of them by amplifled showings.

It is an object of our invention to provide a process of impregnation using thermosetting copolymers as the impregnant, which will more thoroughly impregnate suitable articles than has been hitherto possible.

A further object of our invention is to provide a process of impregnation which will permit a bath of thermosetting copolymer impregnant to be used repeatedly in batch operations.

A leading difiiculty which has arisen in the past in the use of thermosetting copolymers as impregnating or bonding agents has been the tendency of the copolymers to harden prematurely, making their use costly and sometimes impossible, especially when it has been necessary to utilize a large batch of impregnating material, as in a bath for castings, and to continue the bath for a period of time required to obtain satisfactory penetration of the pores of the castings. With some types of plastic coatings for metals, it has been found advantageous to apply a vacuum to the castings prior to the coating bath, in order to remove ultra-minute air particles from the surface and micropores of the metal. But it has not been found practical to use an effective degree of vacuum in conjunction with impregnating materials which harden as their more volatile constituents are driven off, as in thermosetting, because such a degree of vacuum draws ed the volatile constituents as efiectively as they are driven off by heat.

It is not commonly realized that air lodged in the micropores of metal adheres to the metal with a force which tends to nullify or offset the expansion to be expected from Boyles law-a phenomenon which may be verified by applying a vacuum to a bubble caught between threads of colored liquid in a very fine capillary glass. By micropores We mean pores requiring magnification for visibility. To be effective in the micropores of metal and other close-grained or amorphous materials, the absolute pressure at the surface must be much lower than is required merely to detach large air bubbles from the surface. We have found that good results in eliminating air from the pores require an expectancy under Boyles law of around twenty-fold expansion of the occluded air particles, the actual expansion being much less. In terms of pressure, such an expected expansion would correlate to an absolute pressure of 0.735 p. s. i., or to a vacuum of 28.5 inches of mercury related to a 30 inch barometric pressure. Hitherto it has been found that a vacuum of 26 or even 27 inches of mercury, at room temperature or used with warm castings, they are often preheated to remove moisture, would result in so thickening a thermosetting copolymer impregnant of volatile characteristics as to deprive it of capillary penetration into micropores. The longer the impregnating bath lasted under vacuum, the less effective it became, and a batch of impregnant could seldem be used for a second bath. It may here be noted that at a vacuum of 2'? inches of mercury there is theoretically twice as much air left in the micropores as at a vacuum of 28.5 inches, which undoubtedly had an effect in preventing penetration by the impregnant; however, failure of the impregnant to penetrate has generally been ascribed to increased viscosity, as this eiiect was the more obvious.

We have found that'it is possible to utilize a vacuum of 28.5 inches of mercury, or even higher, by cooling a thermosetting copolymer impregnant to a temperature which sufiiciently decreases the vapor tension of the volatile constituents of the impregnant to prevent their escape in material quantity. Only in the absence of escaping vapor is it possible to obtain the high degree of evacuation of the micropores which is essential to their subsequent impregnation. And contrary to the prevalent idea that cooling the impregnant would increase its viscosity and render it less penetrative, we have found that the increase in viscosity due to cooling is more than offset by the maintenance of fluidity attributable to a low rate of evaporation. There is, to be sure, an increase in viscosity attributable to the low temperature, but a much greater loss in fluidity is avoided. And the fifty percent reduction in the density of the residual air in the micropores resultant from increasing the vacuum from an unsatisfactory 27 inches of mercury to 28.5 inches, greatly increases the penetration of the slightly more viscous iinpregnant, both by capillary action and by vacuum suction.

It will be understood that the process herein described is a physical process, capable of acting upon thermosetting copolyniers of diverse chemical organization, and therefore not to be limited to a specific impregnant. Qbviously, it is not applicable to therrnc-plastic impregnants which solidify when cooled, or to solutions dependent on drying by evaporation of volatile solvents rather than by polymerization heat, as such solutions could easily be rendered for repeated baths. Numerous resin compounds are known and available under a wide range of trade narnes which have the desired characteristics that they set by polymerization under heat, and although when in liquid phase they are subject to evaporation, their volatility is, or may be, a subordinate factor in their change to ti a solid state.

An is of a rnio etting copolynier impregnant 1 use with our process an esterification product of an organic polyglycol vith unsaturated. polybasio acid, diluted with monomeric styrene. The dilution may suitably be of the order 20 parts or" the polyester and 30 parts o no one, with a trace of an organic reducing such as hydroguinone. Such an impregnant ainta ned a liquid through lack of cross-l .ge. Prior to its application as a bath, the impregnant is charged with a latent peroxide ca yst, as a consequence of which the equilibrium of the ii'npregnant will be destroyed by heat, a cross-linking pe of polymerization occurring at tempera of 256 degrees Fahrem heit or above.

Impregnants of composition analogous to that stated above therefore responsive to our process are av -ilab commercially under the trade-nan Laminac, Paraplex, Selectron, Thalid, and tilt. An example of a suitable in"- pregnant will be found in the formulations:

the above poiyeste s es being diluted with allyl or styrene nionoiner a ratio dependent upon the viscosity requirement in the general range of T0 parts polyester to styrene, and with the aforesaid trace of organic r ducing agent. It will observed that an impregnant so constituted has no solvents to boil away, but nevertheless will evaporate in vacuo, the liquid styrene being capable of volatiliaing. The ester is only slightly volatile in the viscous mass.

" y out our improved process, we cusheat the castings or other articles to in the usual manner, but allow or room temperature before to the iinpregnant. When cooled, the rticles are placed a suitable vessel, such as an autoclave, capable of withstanding both external and internal pressure and connected to both a vacuum pump and a source of compressed air, and the vessel is closed and exhausted to a pressure of about 0.735 p. s. i. absolute, corresponding to a vacuum of 28.5 inches of mercury. In the meantime, the thermosetting copolymer to be used as an impregnant is cooled to a temperature of 35 to 40 Fahrenheit. The density of the air on the surfaces of the articles having been reduced to approximately one-twen ieth oi the former density, and the density of the air in the inicropores of the articles having been reduced to theoretically the same degree but actually probably to a somewhat higher fraction, the inipregnant is admitted to the vessel which is sti l maintained open to the vacuum pump. -t will be found that the chilled impregnant causes very little increase in pressure within the vessel, indicating that volatile constituents are not liberated to any material degree.

The rticles are permitted to remain submerged in the impregnant bath and under vacuum for approximately ten minutes to ermit the 1:, to absorb minute particles of rari 11 air trapped in the surface or" the aricles to make an intimate and continuous contact with metal, glass, plastic, hard rubber, or whatever material composes the articles. A further advantage of cooling the impregnant here be noted: that a cool liquid has greater affinity for air than the solid articles which will at first be sightly warrior. When sufiicient time has elapsed to assure that the surfaces of the art' s are completely coated without intervening a "holes, the vacuum is broken and a o compressed air is opertively connected t vessel to effect a positive gauge pl'ESSdIG therein. Preferably, the pressure in the vessel raised to betwe n and eight atmospheres, or approximately its 13. s. i. gauge, and maintained at that level for "bout thirty minutes. During this stage the pregnating cycle, the inipregnant is forced into the micropores by pressure and drawn into them by capillary action.

vnen the pressure stage of the cyc e is COlllpleted, the pressure is reduced to -lig atmospheric, and the the vessel to be saved, re-cooled, used again in a subsequent operation, as it will not have lost its fluidity. The vessel may then be opened to the atmosphere an the articles withdrawn and rinsed in a suitable solvent bath to remove excess surface liquid. As a final in the process :ie rinsed articles are baked in a dry oven to attain a temperature of approx 'natel" Sal-3 and then allowed to cool. The virtual complete absence or" air in the micropores causes the impregnant to be retained in the nricropores during the rinsing and baking steps.

The apparatus required to carry our process into effect being all standard and used in manners customary for the ir pieces, it has not been believed necessary to describe or to illustrate it in detail, as suitable arrangements 01" the apparatus will be obvious to those skilled in the art.

We claim:

The process microscopic pores; ent for hardening upon polymerizat' influence or" heat as p v tile component a liquid styrer consists of: subjecting the at ticle to a pressure not substantially exceeding 0.735 p. s. i. absolute; cooling the impregnant to a temperature at which the vapor tension oi said styrene monomer at 6.735 p. s. i. absolute is substantially equivalent to its vapor tension at room temperature and atmospheric pressure; submerging said article in a bath of said cooled i'mpregnant, while maintaining the pressure in the order of 0.735 p. s. i. absolute; then subjecting the submerged article to a pressure in excess of six atmospheres; releasing said super-atmospheric pressure and removing said article from said bath; and polymerizing the impregnant residual on said article by heat-treatment.

References Cited in the file of this patent UNITED STATES PATENTS Number 

